There are a number of techniques for removing the bark from logs.
Probably the best known debarkers are those which use rollers carrying teeth on their outer surfaces. The teeth are designed and positioned to rip pieces of bark from a log passing between the rollers until all the bark is removed. Examples of such debarkers are described in the specifications of U.S. Pat. Nos. 3,363,720 (to G. W. Brock and H. J. Merrifield) and 3,587,685 (to N. K. Morey and L. N. Smith).
A similar approach to debarking is found in the drum debarkers. With a drum debarker, logs are passed through a rotating drum which has internal vanes or ribs. As the logs are tumbled within the drum, they strike each other and the vanes or ribs within the drum, and these impacts act to tear the bark from the logs. Descriptions of drum debarkers are found in the book entitled "Handbook of Pulp and Paper Technology" (second edition), edited by K. W. Britt and published by Van Nostrand Reinhold Company, at pages 103 and 122, and also in the book entitled "Forest Products, their Sources, Production and Utilization", by Panshin, Harrer, Bethel and Baker, published by McGraw-Hill Book Company (second edition, 1962), at pages 332 to 335. Such drum debarkers are unsuitable for debarking stringy-bark eucalypts and other species in Australia.
The rotary or ring debarkers--a number of models have been used--cut the bark with a knife before it is stripped from a log. A typical ring debarker has a hollow rotor unit within which are mounted inwardly projecting cutters or barking tools. The cutters or barking tools are rotated about a log as it is moved through the rotor to cut or strip the bark from the log. Examples of developments in rotary debarkers are found in the specifications of Australian patents Nos. 479,105 (to Kokum Industri Aktiebolag) and 501,776, (to V. L. Valo), and Australian patent applications Nos. 21267/83 (Hutson) and 66304/86 (Fuji Kogyo K K).
A third debarking technique, known as compression debarking, which is suitable for removing bark from stringy-bark eucalypts, involves the application of substantial pressure to the bark of a log, to break the bond between the bark and the wood, and leave a tube of bark surrounding, but separated from, the wood. The tube of bark is then cut into strips by a knife, which is typically mounted on a roller downstream from the pressure-applying rollers, although the cutting of the bark may occur before, during or after the application of pressure to the bark. Examples of this type of debarker are described in the specification of Australian patent application No. 82427/87, now Australian Patent No. 604,514 (in the name of Commonwealth Scientific and Industrial Research Organisation) and in the specification of the corresponding U.S. patent application Ser. No. 131,485 filed Dec. 10, 1987, now U.S. Pat. No. 4,875,511.
Other debarking techniques include hydraulic barkers using water jets under high pressure, flails (see the specification of Australian patent No. 506,204 to L. J. Emmins), "knocking" the bark from a log (see the specification of Australian patent No. 511,333 to G. A. Williames), and, of course, manual removal of the bark with axes and the like.
The present invention concerns the third debarking technique referred to above, compression debarking. Two recent proposals for an improved approach to debarking by this technique were the subject of the aforementioned Australian patent application No. 82427/87, in the name of Commonwealth Scientific and Industrial Research Organisation. One of those proposals (excluded from the specification by amendment and thus not featured in the specification of Australian Patent No. 604,514) involves the passage of a log through the open-ended passage defined by three rollers which are mounted on a frame with their axes of rotation arranged angularly relative to each other and to the elongate direction of the passage. Thus, instead of being parallel to each other, the axes of the rollers are skewed relative to the elongate direction of the passage and the rollers form a tapered or converging passage. The rollers all rotate in the same direction, and at least two of the rollers are mounted resiliently on their support frame, to enable the cross-sectional dimension of the passage to change to allow logs of various size to be debarked. The cylindrical outer surface of each roller is grooved or is otherwise roughened, or is provided with welded-on bars or the like. The rollers may be rotated at different speeds.
A log that is to have its bark removed is fed endwise into the passage and passes through it under the influence of the grip on the log by the outer surfaces of the rollers combined with the rotation of the rollers. As the log is drawn through the tapered passage, it is subjected to increasing radial compressive forces, and these forces cause the bark to separate from the wood of the log. One of the rollers is provided with a circumferential "knife" on its outer surface, to cut through the loose bark which tends to remain as a tube around the body of logs of rough, stringy-barked eucalypts and the like.
The second proposal disclosed in the specification of the aforementioned Australian patent application No. 82427/87 (now the sole subject of Australian Patent No. 604,514 and U.S. Pat. No. 4,875,511) involves the provision of at least two pairs of rollers, each pair mounted with the axes of its rollers parallel to each other, and at an angle to the axial direction of the other pair of rollers. The outer surface of each roller is concave, and the pairs of rollers are mounted in spaced apart relationship along the path followed by a log--one pair of rollers thus being effectively downstream of the other pair or pairs. A log to be de-barked is passed between the rollers of the first pair, then through the second pair of rollers, and then between other pairs of rollers if present. The rollers of at least the first pair are grooved or otherwise roughened to provide a firm grip on a log. Each roller of the final two pairs of rollers is provided with one or more helically-positioned knife blades on its outer surface.
When a log to be debarked is fed into the first pair of rollers, the first rollers apply a compressive force to the log to such an extent that the bond between the bark and the wood is broken and the bark separates from the timber body of the log. The bark remains as a tube of enlarged cross-section around the timber body. Knife blades affixed to the rollers cut the tubular bark into strips, which fall from the log. This arrangement is effective with logs having a substantially uniform cross-section with a diameter approximately equal to twice the radius of curvature of the concave shape of the rollers. However, it exhibits problems when the logs are not essentially circular in cross-section and when the logs have large branch stubs and other irregularities, which tend to block the forward movement of the log through the pairs of rollers.